Only for the cMP of course, because the nMP is about as expandable as a singularity. The nMP already has USB 3 (x4) as well as 2x faster TB2 (x6) and there are adapters for both to USB C. Upgrades are only needed to upgrade to something a computer doesn't have. This USB C is data only too, not with displayport like on the Mac laptops so no plugging in a display.
People needed RAM upgrades when there was under 4GB of RAM, HDD upgrades when storage was below 128GB, battery replacements when battery life was under 5 hours, IO upgrades when we just had 800Mbits or less. None of this is true any more. Machines ship with 8-16GB RAM with memory compression, SSDs are now 2-3GB/s and 256GB+, batteries last 10 hours, the latest IO is 40Gbits with display output.
'does work on the 3,1, 4,1 and 5,1 Mac Pro, and can deliver a full 10 gigabits per second transfer speed from each port.' Mike Wuerthele, I'd like proof of this. Most of what has been written about the ASM1142 based cards is that they do work in Sierra but only at 5Gbps. Many Mac computers have USB-A ports (sometimes referred to as USB 3 ports), which look like this: USB 3.0 and USB 3.1 Gen 1 can transfer data up to 5 Gbps, and USB 3.1. The MAXPower USB 3.1 Gen 1 – 4-port PCIe card gives you more options. Four more options, in fact. Four high-power, high bandwidth USB ports for Mac Pro and PC with maximum wattage and data throughput, for a total of up to 5 Gb/s and 15W.